Over the last two decades, research on student achievement has pinpointed the central role of teachers. There is a general consensus that improving the effectiveness of teachers is the key to lifting student achievement, although questions remain about how best to do this. A key element in focusing attention on the importance of teacher effectiveness was research that took an outcomes-based perspective. By looking at differences in the growth of student achievement across different teachers instead of concentrating on just the background and characteristics of teachers, it was possible to identify the true impact of teachers on students. This work, now generally called value-added analysis, demonstrated that some teachers consistently get greater learning gains year after year than other teachers. For the sake of K-12 students, we need to validate and reward our best instructors and get rid of our worst ones.